Foreign Object Damage:

by Charles Chandler On May 15, 2009

On Jan.15, 2009, on climb out from KLGA at about 3,200 feet, US Airways Flight 1549 encountered a bit of FOD, or foreign object damage, most likely a flock of migrating Canada Geese. The comment from the cockpit was “Aaah, this is Cactus 1549. We hit birds. We lost thrust in both engines. We’re turning back toward LaGuardia.” The next series of events concluded with the Captain Chesley Burnett Sullenberger III, First Officer Jeffery B. Skiles, and cabin crew Sheila Dail, Doreen Welsh, and Donna Dent making a successful water landing in the Hudson River.

On that January morning the good passengers on Flight 1549 were holding a fist full of aces. They were ever so fortunate to be flying with Captain Sully. I think all of us have thought that if we ever have to ditch, we want Captain Sully to be up front. He is an expert in crew resource management (CRM) and aircraft safety, and a master of the psychology in keeping an airline crew functional during just such events. Captain Sullenberger was a fighter pilot, holds an Airline Transport Pilot Certificate and Commercial Pilot Certificate, and is an expert flight instructor for gliders. This experience was extremely important after the engines stopped, and it became very quiet up front. Captain Sully successfully negotiated the morning rush hour on the Hudson and put the plane down in the ferry route between New York and New Jersey. All 150 passengers and crew were evacuated safely.

That was the happy ending — and now the aircraft and engines are behind closed doors, and forensic detectives continue the technical investigations. Volumes of data will be produced, reviewed, and analyzed before the final NTSB report is written. It will probably be a couple of years before we can read the technical details from this bird strike. However, we can advance the Flight 1549 story a chapter or two with some existing information from the original equipment manufacturers (OEMs), preliminary data from the agencies, and our own knowledge of history.

Aviators and aircraft mechanics have a long and troublesome history with bird strikes. According to his log, on Sept. 7, 1908, Orville Wright’s aircraft hit and killed a bird while flying circles over a field near Dayton, OH. On April 3, 1912, Cal Rodgers, the first person to fly across the country (with Charles Taylor as his mechanic), was flying an exhibition near Long Beach, CA, where it was reported that he struck a seagull, jamming the flight controls. He was killed when his Model EX “Vin Fiz” crashed into the breakers. Data today suggests that the annual cost to the aviation community from bird strikes is upward of $1.2 billion in direct repair cost, as well as lost revenue opportunities to owners and operators. There is no cost that can be assessed to the lives lost from these events.

I was specifically interested in the damage to the engines caused by the bird strikes. Preliminary NTSB forensic evidence on Flight 1549 suggests that the “primary targets” were Canada Geese. Damage from bird strikes begins at the point of impact, usually on the leading edges of the wings and stabilizers, engine nose cones, cowlings, and inlets. Engine ingestion of objects can be catastrophic because of the stacked modular design of the engine, and the energy developed by the rotating mass of the compressors and turbines.

FOD to engines in cases such as bird strikes can trigger succession damage, leading to tiny nicks on the front edges of the fan blades, to full-scale destruction of the compressor and turbine section. Usually when the FOD strikes fan blades, the blades distort and cause tip scrub and curling, as well as the loss of material from the casing. This abrasion can continue into the low pressure compressor, resulting in more damage and debris that can affect the leading edges of the high-pressure compressor blades and vanes. If the damage is great enough to disrupt airflow, the engine loses thrust or may seize completely. Aircraft are likely to incur the most damage during takeoff and climb out when power settings are high, the plane is at a low altitude, and crews have little time and room to maneuver.

The power plants
The equipment used on US Air Flight 1549 was a narrow body Airbus 320 (A320) powered by two CFM56-5A engines. The cases on these tough engines contained the FOD damage as designed, resulting in no collateral damage to the fuselage from disk and blade shrapnel. I was able to discuss the CFM56 engine with Deborah Case of General Electric (GE) Aviation. The CFM56 Series engines are used on commercial, corporate, and military aircraft, powering more than 1,706 aircraft worldwide, and accumulating more than 46 million flight hours in the process.

The CFM56 Series engines are built by CFM International, a 50/50 joint company of GE and Snecma (SAFRAN Group) of France.

The CFM56 engines on the A320 were certified by both the European Joint Airworthiness Authority (JAA) and, subsequently, by the FAA. The engines were originally certified to take a hit in the core engine from a flock of seven 1.5-pound birds and run for five minutes at a takeoff power setting. Current standard for the CFM56-5 is a hit from one 2.5-pound bird followed by five 1.5-pound birds, with a maximum allowable 25 percent loss of thrust. CFM exceeded these requirements, and actually tested the engine using three 2.5-pound birds. The CFM56-5 engines on Flight 1549, even though tested to the higher standard, still failed that morning in January. Tough as they are, bird strikes are problematic for all aircraft engines. Engine failure caused by bird strikes that result in complete loss of power is relatively rare. This seems like a contradiction, but not so because engines are designed to withstand bird strikes.

It is this Flight 1549 failure that is most concerning to OEMs, pilots, passengers, and the regulatory agencies. The FAA recently amended the certification standard for future engines by raising the weight of the bird to 8 pounds. Some of the feedback to the FAA suggested that weight is too low. Some argue that the weight should be raised to a realistic level that would include birds as large as the Canada Geese that brought Flight 1549 down in the Hudson.

It was obvious from news media footage and statements from the NTSB that both of Flight 1549’s engines were a mess. Large pieces of the cowlings were missing, and on initial examination the NTSB reported that while missing obvious organic matter (code for Canada Goose liver pate), they had found evidence of soft body impact, and “found dents on both the spinner and inlet cowling.” In addition, the inlet guide vanes were fractured, and eight outlet guide vanes were missing. Both engines have been sent to the GE Aviation facilities in Cincinnati, OH, for disassembly, examination, and analysis. Having seen the damage bird strikes can cause to jet engines, I know the nondestructive testing (NDT) crews will be busy for quite some time while poring over these two engines.

The ongoing NDT inspections
If you have read the March AMT article on the “Effective Use of Borescopes” by James Careless, you know there are some great new NDT technologies out there. While it can’t be known which inspection equipment was being used for the US Airways plane, the actual technologies and applications will depend on the distinctive characteristics of the accident and the specific components under inspection.

Most likely, the investigation will include remote visual inspection (video borescopes) and ultrasound, as well as eddy current and digital X-ray to ensure that as much data as possible can be gleaned from this accident. Jeff Anderson, aerospace expert at GE Sensing & Inspection Technologies, points out that composite structures do not necessarily retain impact damage marks and was concerned about the extensive damage that the A320 airframe and engines had already sustained. He was well aware that the inspection crews must be careful not to further stress those areas like the pylons, engine mounts, and adjacent fuselage. These areas will also hold data critical for understanding the consequences of the bird strike and engine failures of Flight 1549.

To help in the investigation process, the FAA made its entire bird strike database available on a public website Friday, April 24. The FAA has determined that it can release the data without jeopardizing aviation safety. Over the next four months, the FAA will make changes to improve the search function and make it more user-friendly. It also plans to work with the aviation community to find ways to improve and strengthen bird strike reporting.

Inspections are the final phases in the process of an ongoing aviation accident investigation. We don’t have the full NTSB report at this time, but hopefully it will hold clues for us all to use to try to prevent this kind of accident in the future. All of us who are maintenance technicians have the responsibility to ensure the airworthiness of our aircraft, and we need all the help we can get to fight the good fight against our old enemy FOD.

Charles Chandler is AMT’s Field Editor. A Texas-based A&P, he received his training at Spartan College of Aeronautics.